Super Smash Brothers Melee
Secret characters

Jigglypuff:  Clear Classic or Adventure mode with any character.

Luigi: Beat the first level in Adventure mode with a 2 in the seconds place, (eg. XX:X2:XX)  Then Luigi and Peach will challenge you instead of Mario and Peach.

Young Link: Clear Classic with 10 characters including Zelda and Link or clear 500 vs. Matches.

Dr. Mario:  Clear Classic or Adventure with Mario without using a continue

Gannondorf:  Clear event Match 29

Pichu: Clear 200 vs. matches or clear Event match 37

Marth:  Clear 400 vs. matches or use each default character in a vs. match at least once.

Roy:  Clear adventure mode with Marth without using a continue

Falco: Clear 300 vs. matches or beat 100 man melee

Mewtwo:  Get 20 hours combined play time  (Combined meaning, if there were 2 human players, and you played for 2 minutes, combined play time would add 4, 1 human would add only 2)  Or complete 700 vs. matches.

Mr. Game and Watch:  Break the targets, beat Adventure mode, or Classic Mode with all the characters, excluding Mr. Game and watch (well duh!)